In the automatic testing of electrical circuits, test probes of various configurations are used depending upon the type of electrical device under test. A type of test probe commonly referred to as a "switch probe" is used in the automatic electrical continuity testing of electrical equipment. Cable harnesses for automobiles are one example. Such a switch probe is used in a two-level test. The first level is a "push-out" test for testing the mechanical integrity of mounting a conductor terminal inside a plastic connector housing. If the terminal is mounted correctly, the switch closes. If the terminal is not seated correctly in the connector body, the terminal is pushed out of the connector housing and the switch does not close. The second level is an electrical continuity test.
A typical switch probe is a miniature-sized device which includes a barrel, an insulator, a terminal in one end of the barrel isolated by the insulator, a movable plunger projecting from the opposite from the end of the barrel, and a coil spring inside the barrel between the plunger and the terminal. All components are contained within an outer receptacle. A downward force applied to the plunger against the bias of the spring moves the plunger toward a switch contact point on the terminal inside the probe.
It is a difficult problem to manufacture the miniature-sized components of such switch probes while also maintaining the necessary reliable operation from cycle-to-cycle over an extremely large number of operating cycles. For instance, plunger travel inside the switch probe is particularly critical, and if the probe has a slight amount of over-travel or under-travel, the reliability of the switching process suffers. In one prior art switch probe, the critical closure travel distance is controlled by a plastic insulator that spaces the conductive elements of the switch from each other and holds active components of the switch together. The plastic insulator is a joint which holds two metal parts of the terminal barrel assembly together. Plastic does not always hold well under axial pressure because of its cold flow tendencies. In the switch assembly process, a "roll close" is used for closing the assembled parts. This produces a crimping action which can cause the plastic to cold flow. Any plastic flow can change the critical distance of switch closure travel. Switch probes with a plastic insulator also are limited in their use in thermal environments.
In these prior art switch probes, it is difficult to control tolerances so that the critical closure travel is not adversely affected. For instance, in one type of switch probe, the critical switch travel from the normally open to the closed position of the switch is about 0.025 inch. As components of the switch are assembled together, tolerances build up. Tolerances on the closure travel distance from such build-up of tolerances from the various assembled parts can be in the neighborhood of about 0.012 to about 0.015 inch. With such a wide variation in tolerances as a percentage of the specified switch closure travel, the reliability of the switch probe during use is a major problem.
The present invention provides a switch probe which greatly reduces the tolerance problems and problems of reliability associated with the prior art switch probes described above. The switch probe is also greatly simplified with fewer and less complicated parts and therefore has better control over tolerances that affect the critical switch closure distance. Plastic cold flow is avoided and use in thermal environments is not restricted.